Made podman preferred backend (#22)

Author: ssbarnea

Diff for: README.rst

@@ -21,9 +21,12 @@ Molecule Containers is designed to be a **drop-in replacement for the existing
 Docker and Podman drivers**, one that can transparently pick whichever backend
 is found.
 
-Please note that this driver is currently in its **early stage of
-development**, do not even try to use it in production. Just raise a PR if you
-have a bugfix for it.
+The driver preference is defined by
+``MOLECULE_CONTAINERS_BACKEND=podman,docker`` and you can easily switch between
+the two by setting this variable.
+
+Keep in mind that if executable is not found, the driver will fallback to
+first option in the list and likely fail later.
 
 .. _get-involved:
 

Diff for: molecule_containers/driver.py

@@ -9,19 +9,13 @@
 
 _logger = logger.get_logger(__name__)
 
-# Temporary logic to determine which backend driver should be loaded.
-# podman is used as fallback default.
-driver = os.environ.get("MOLECULE_CONTAINERS_BACKEND", "")
-if not driver:
-    if "DOCKER_HOST" in os.environ:
-        driver = "docker"
-    elif "CONTAINER_HOST" in os.environ:
-        driver = "podman"
-    else:
-        if shutil.which("docker"):
-            driver = "docker"
-        else:
-            driver = "podman"
+# Preference logic for picking backend driver to be used.
+drivers = os.environ.get("MOLECULE_CONTAINERS_BACKEND", "podman,docker").split(",")
+for driver in drivers:
+    if shutil.which(driver):
+        break
+else:
+    driver = drivers[0]
 
 # Logic for picking backend is subject to change.
 if driver == "docker":